1. Field of the Invention
The present invention relates to a quartz crystal unit in which a quartz crystal blank is held on a metallic base, and more particularly, relates to a crystal unit improved in its shock resistance.
2. Description of the Related Art
A crystal unit is incorporated in an oscillator as a reference source for frequency, and is adopted by various kinds of telecommunication equipment. As one of these crystal units, there is a certain crystal unit employing a quartz crystal blank of AT-cut or SC-cut to increase stability.
FIGS. 1 and 2 are a cross-sectional view and a plan view, respectively, illustrating the constitution of a conventional crystal unit. Generally, a crystal unit holds crystal blank 3 above metallic base 1, and then covers metallic base 1 with a metallic cover to thereby seal crystal blank 3 in the metallic container consisting of metallic base 1 and the metallic cover. FIGS. 1 and 2 are views illustrating a state where the metallic cover is not provided.
In the illustrated crystal unit, crystal blank 3 is of substantially circular plate, and its circumferential portion is equiangularly held by four L-letter-shaped metallic brackets 2a, 2c, 2b and 2d provided on metallic base 1. Four lead wires 4a, 4c, 4b and 4d arranged to be in registration with brackets 2a, 2c, 2b and 2d are disposed so as to penetrate the base body portion of metallic base 1. In these lead wires, at least a pair of opposing lead wires 4a and 4b are provided with, at their portions penetrating the base body portion, non-illustrated glass portions by which lead wires 4a and 4b are electrically insulated from the base body portion. That is, at least the pair of opposing lead wires 4a and 4b are provided as hermetic terminals against metallic base 1.
Any one of L-letter-shaped metallic brackets 2a through 2d is provided with a vertical portion and a horizontal portion, and the horizontal portion is spot-welded by, for example, laser beam to the extreme end of each of lead wires 4a through 4d. Brackets 2a and 2b connected to lead wires 4a and 4b have their horizontal portions which extend toward the central portion of metallic base 1, and their vertical portions which are arranged on the side of the outer circumference of metallic base 1 while permitting the inner faces thereof to diametrically confront each other. Similarly, brackets 2c and 2d connected to lead wires 4c and 4d have their horizontal portions which extend toward the central portion of metallic base 1, and their vertical portions which are arranged so that the inner faces thereof diametrically confronting each other.
Crystal blank 3 of circular plate consists of, for example, a crystal blank of AT cut, and is formed with excitation electrodes 5a and 5b on both of the principal planes thereof. Further, extending electrodes 6a and 6b are provided to extend from excitation electrodes 5a and 5b toward the outer circumferential portion of crystal blank 3 in mutually opposite directions. Extending electrodes 5a and 5b are formed so as to be folded back from one to the opposite face at the edge of crystal blank 3. Further, extending electrodes 6a and 6b are electrically and mechanically connected to brackets 2a and 2b connected to lead wires 4a and 4b at positions of the end faces of crystal blank 3. In this case, a metallic foil is disposed between a portion of respective extending electrodes 6a and 6b, which appears at the end face of crystal blank 3, and the inner face of the vertical portion of each of brackets 2a and 2b, so as to be bonded there by thermo-compression. In other words, the bonding is achieved by the so-called bracing. Furthermore, the vertical portions of brackets 2c and 2d are similarly bonded to the end face of crystal blank 3 by the method of bracing. Brackets 2c and 2d are not electrically connected to excitation electrodes 5a and 5b, and merely exhibit a function for holding crystal blank 3.
In this type of crystal unit, since crystal blank 3 is mechanically connected, at four positions thereof, to brackets 4a through 4d, relatively large crystal blank 3 can be stably held. Further, the end face of crystal blank 3 is used for connection, but no connecting material is attached to the principal planes thereof. Thus, a good vibration characteristic of the quartz crystal blank can be maintained. Also, since any organic material such as conductive adhesive is not employed, a good aging characteristic of the crystal unit can be ensured. From these reasons, this type of crystal unit has been suitably used for, for example, a crystal oscillator with high stability, which employs a thermostatic bath.
Nevertheless, in the above-described crystal unit, brackets 2a through 2d are connected to lead wires 4a through 4d by point connection by using the spot welding. Therefore, the strength of connection of brackets 2a through 2d to lead wires 4a through 4d is small, and accordingly when any shock by falling or the like is applied to the crystal unit, rotational moment acts about the end of each of lead wires 4a through 4d. As a result, there occurs a problem such that a stress is generated in crystal blank 3 to cause a change in the resonance frequency before and after the application of the shock.
Also, respective brackets 2a through 2d are individually connected to lead wires 4a through 4d, and thus, an operation to conduct positional alignment during the production of the crystal unit must have become rather difficult. Particularly, since it is necessary for a distance between mutually opposed brackets 2a and 2b, and a distance between mutually opposed another pair of brackets 2c and 2d to be made equal, the positional alignment takes long time to thereby bring about a reduction in fabrication performance and productivity.
An object of the present invention is to provide a crystal unit especially improved in its shock resistance.
Another object of the present invention is to provide a crystal unit capable of easily performing a positional alignment operation during the production process while maintaining the shock resistance of the crystal unit.
The above objects of the present invention can be achieved by a crystal unit, which comprises a metallic base, a crystal blank having first and second principal planes, first and second excitation electrodes respectively formed in the first and second principal planes, a first extending electrode extending on the first principal plane from the first excitation electrode in a first direction toward an outer circumferential portion of the crystal blank, a second extending electrode extending on the second principal plane from the second excitation electrode in a second direction different from the first direction toward the outer circumferential portion of said crystal blank, first and second L-letter-shaped metallic brackets respectively electrically and mechanically connected to the first and second extending electrodes at the outer circumferential portion of the crystal blank, first and second lead wires constituted as hermetic terminals penetrating the metallic base and bonded respectively to the first and second brackets, third and fourth L-letter-shaped metallic brackets mechanically connected, at positions different from connecting positions of the first and second brackets, to portions of the outer circumferential portion of the crystal blank, in which a naked surface of the crystal blank is exposed, the third and fourth brackets being interconnected to form an integral metallic holding member, and means for connecting the holding member to said metallic base.
In the present invention, since the third and fourth L-letter-shaped metallic brackets are interconnected to form the integral holding member, the third and fourth brackets are mutually secured, and therefore even if the holding member is bonded by spot welding to the ends of the two lead wires, any rotational moment generated by shock or the like is attenuated or mitigated to result in an improvement of the shock resistance of the crystal unit.
Further, in the crystal unit according to the present invention, the number of brackets, which require a positional alignment operation is substantially reduced compared with the conventional crystal unit, and therefore the operation for the positional alignment can be easier.